Tools have been remotely triggered in the past by a variety of ways. One way shown in U.S. Pat. No. 6,382,234 is to use an electric heater to melt a plug that then opens a flow port to allow an actuating piston to displace. In this device the actuating piston is not mechanically restrained, rather fluid is retained by a plug. As long as the fusible plug is intact the fluid ahead of the piston has nowhere to go. When heat melts the plug the fluid can be displaced as the setting piston responds to a spring force unleashed by the fluid ahead of the piston having a place to be displaced.
Another design shown in U.S. Pat. No. 7,819,198 holds a coiled spring in a wound state around an actuator. A wire holding the spring and surrounding housing over the actuator is melted which allows the spring to radially displace the components retaining the actuator radially so that the actuator can move axially to set a tool.
The latter design stacks components radially which dramatically increases the diameter of the lock for the tool actuator. In some applications space is simply not available for such a bulky lock mechanism. Melting a fusible plug as in the former design also requires a great deal of power to generate the heat needed to defeat the fusible plug. There are further uncertainties with the degree of melting that insures the ability to displace enough fluid at the needed rate to get the ultimate borehole tool to set.
What is needed and provided with the present invention is a low profile design that aligns the mechanical restraint axially with the lock elements and the shaft or pin that needs to move to get the tool set either by opening a port to take advantage of available hydrostatic or to move an actuation rod when the available hydrostatic may be insufficient to actuate the borehole tool. These and other aspects of the present invention will be more readily apparent from a review of the description of the preferred embodiment and the associated drawing, while recognizing that the full scope of the invention is to be determined from the appended claims.